A conventional window lock as shown in FIG. 7 includes: a semicircular disk pivotally secured on a first window leaf W having a handle H and an extending rim R engageable with a hook H1 formed on a second window leaf W1 for locking the pair of window leaves W, W1 when closed to each other. Since the conventional window lock can be easily discovered by an intruder or thief outside the window, he may then cut out a small opening C on the window W or W1 for easily unlocking the conventional window lock.